A type of such control apparatuses set forth above is designed to carry out current feedback control to thereby adjust an actual torque of a rotary machine to a request torque. A typical control apparatus of this type operates in a PWM (Pulse Width Modulation) control mode for switching elements of an inverter as an example of power converters.
The control apparatus for a three-phase motor operates in the PWM control mode to calculate a substantially sinusoidal command voltage for each phase winding of the three-phase motor; this command voltage is required to match an actual current flowing through each phase winding and fed back therefrom with a desired periodic command current.
The control apparatus operates in the PWM control mode to compare the sinusoidal command voltage for each phase winding with a triangular (or saw-tooth) carrier wave. Based on the result of the comparison, the control apparatus operates in the PWM mode to individually switch on and off each of bridge-configured switching elements of an inverter based on the result of the comparison. This modulates an input DC voltage to the inverter into an AC (Alternating Current) voltage to be applied to each phase winding of the rotary machine.
Adjustment of the on and off durations, that is, the duty (duty cycle) of each of the bridge-configured switching elements by the control apparatus matches the AC voltage to be applied to each phase winding with the command voltage therefor. This matches the actual current flowing through each phase winding to a desired periodic command current. The actual current flowing through each phase winding works to generate a torque corresponding to the desired command current for each phase winding.
The PWM control mode for a three-phase motor needs to increase the command voltage in a higher speed range of the three-phase motor. The bridge-configured inverter limits an upper limit of the amplitude of the command voltage to substantially the half of the input DC voltage to the inverter. This is because the substantial half of the input DC voltage to the inverter is applied to each phase winding.
Thus, when the command voltage increases in amplitude to be greater than the half of the inverter input DC voltage, an actual output voltage of the inverter cannot be matched with the command voltage.
Thus, in a higher speed range of a three-phase motor, using a single-pulse control mode in place of the PWM control mode has been implemented.
A control apparatus operates in the single-pulse control mode in a higher speed range of the three-phase motor to individually switch on and off each of the switching elements of the inverter such that the on and off cycle of each of the switching elements is substantially matched with the period of the periodic command current; this period corresponds to an electric angle of 2π radians.
The control apparatus that operates in the single-pulse control mode in a higher speed range of the three-phase motor provides a voltage utilization factor greater than that obtained when it operates in the PWM control mode in the higher speed range. The voltage utilization factor means the ratio of a magnitude of an inverter output voltage to the inverter input DC voltage.
However, the single-pulse control mode abruptly, that is, discontinuously increases the voltage utilization factor from the value obtained at the moment when the amplitude of the command voltage for the PWM control mode reaches the half of the input DC voltage to the inverter.
An additional control method for continuously shifting inverter control from the PWM control mode to the single-pulse control mode is disclosed in Japanese Patent Application Publication No. H09-047100.
The method disclosed in the Patent Publication is designed to, when the amplitude of the command voltage for the PWM control mode reaches the half of the inverter input DC voltage, use a pattern of periodic repetitive pulses stored in a ROM and a phase of a vector of the command voltage in a d-q coordinate system. The d-axis of the d-q coordinate system is in line with a rotor N pole center of the three-phase motor, and the q-axis thereof has a phase of π/2 radian electric angle leading with respect to a corresponding d-axis during rotation of the three-phase motor.
The method is also designed to switch on and off each of the bridge-configured switching elements in accordance with the pattern of periodic repetitive pulses.
This makes possible that the voltage utilization factor obtained at the moment when the amplitude of the command voltage for the PWM control mode substantially reaches the half of the inverter input DC voltage is continuously shifted to the voltage utilization factor obtained using the single pulse control mode.